Dispersoid-free zones in the heat-affected zone of aluminum alloy welds
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I. INTRODUCTION
WELDING is the most important method of joining components made of metallic materials. The welding heat always has a major influence on the microstructure of the material. The mechanical properties of welded metals reach, only in a limited number of cases, the properties of the base material. Welding of aluminum and its alloys is more difficult than welding of iron-based alloys, because of the high thermal conductivity and the large thermal-expansion coefficient of aluminum. It is further complicated through the oxide layer that is rapidly formed on the surface of all aluminum-based metals and which has a very high melting point (2320 K). However, welding of pure aluminum and nonheat-treatable aluminum alloys is today an established technology.[1] In contrast, heat-treatable aluminum alloys still cannot be welded without a significant loss in mechanical properties compared to those of the base material.[2] This becomes even more evident when the base material is tuned to the highest possible strength through the precipitation process. For those alloys, the degradation in strength in the vicinity of the weld is profound, resulting in decreased mechanical properties. The general development of the microstructure in heat-treatable aluminum alloys at different distances from the center of the weld is described in the following text. The welding microstructure is generally divided into three well-defined zones called the “fusion zone” (FZ), the “heataffected zone” (HAZ), and the “base material” (BM). Those zones are demonstrated by the hardness profile in Figure 1. Each zone has experienced its own time-temperature profile, which is shown schematically in Figure 2. In the FZ, the material has been completely melted. The microstructure is a typical solidification microstructure with dendrite or cellular morphology. Aluminum cells are surrounded by a seam of low-melting-point eutectic, while a grain in the FZ is larger and consists of a number of cells. The boundary between the FZ and the HAZ is termed the “fusion line” (FL). B.C. MEYER and G. TEMPUS, Material Specialists, H. DOYEN and D. EMANOWSKI, Material Specialists, are with DaimlerChrysler Aerospace Airbus GmbH, 28199 Bremen, Germany. T. HIRSCH, Material Specialist, Physilial Analytik, and P. MAYR, Head, are with the Institute fu¨r Werkstofftechnik, 28199 Bremen, Germany. Manuscript submitted March 11, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
In the HAZ, the heat of the welding process causes microstructural changes. Those changes may affect the grain structure or, in heat-treatable alloys, the metastable strengthening phases. Stable phases, such as the dispersoids,* are generally *“Dispersoids” are stable AlMn(Fe)Si phases which precipitate during an ingot preheat and control the recrystallization behavior of the alloy. They are not to be confused with the finely dispersed insoluble phases of dispersion-hardened alloys.
not affected. In heat-treatable alloys, the HAZ can further be divided into the HAZs 1 through 3 (Figure 1). The temperatu
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